to be seen. To carry on the Computation, let us therefore suppose that
the Differences of the Diameters of the Circles made by the outmost red,
the Confine of red and orange, the Confine of orange and yellow, the
Confine of yellow and green, the Confine of green and blue, the Confine
of blue and indigo, the Confine of indigo and violet, and outmost
violet, are in proportion as the Differences of the Lengths of a
Monochord which sound the Tones in an Eight; _sol_, _la_, _fa_, _sol_,
_la_, _mi_, _fa_, _sol_, that is, as the Numbers 1/9, 1/18, 1/12, 1/12,
2/27, 1/27, 1/18. And if the Diameter of the Circle made by the Confine
of red and orange be 9A, and that of the Circle made by the Confine of
blue and indigo be 8A as above; their difference 9A-8A will be to the
difference of the Diameters of the Circles made by the outmost red, and
by the Confine of red and orange, as 1/18 + 1/12 + 1/12 + 2/27 to 1/9,
that is as 8/27 to 1/9, or 8 to 3, and to the difference of the Circles
made by the outmost violet, and by the Confine of blue and indigo, as
1/18 + 1/12 + 1/12 + 2/27 to 1/27 + 1/18, that is, as 8/27 to 5/54, or
as 16 to 5. And therefore these differences will be 3/8A and 5/16A. Add
the first to 9A and subduct the last from 8A, and you will have the
Diameters of the Circles made by the least and most refrangible Rays
75/8A and ((61-1/2)/8)A. These diameters are therefore to one another as
75 to 61-1/2 or 50 to 41, and their Squares as 2500 to 1681, that is, as
3 to 2 very nearly. Which proportion differs not much from the
proportion of the Diameters of the Circles made by the outmost red and
outmost violet, in the 13th Observation of the first part of this Book.
_Obs._ 6. Placing my Eye where these Rings appear'd plainest, I saw the
Speculum tinged all over with Waves of Colours, (red, yellow, green,
blue;) like those which in the Observations of the first part of this
Book appeared between the Object-glasses, and upon Bubbles of Water, but
much larger. And after the manner of those, they were of various
magnitudes in various Positions of the Eye, swelling and shrinking as I
moved my Eye this way and that way. They were formed like Arcs of
concentrick Circles, as those were; and when my Eye was over against the
center of the concavity of the Speculum, (that is, 5 Feet and 10 Inches
distant from the Speculum,) their common center was in a right Line with
that center of concavity, and with the hole in the Window. But in other
postures of my Eye their center had other positions. They appear'd by
the Light of the Clouds propagated to the Speculum through the hole in
the Window; and when the Sun shone through that hole upon the Speculum,
his Light upon it was of the Colour of the Ring whereon it fell, but by
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